Over the years, things have gotten easier for potters. We don’t have to find and dig our own clay, transport it to our studio on a mule, slake it down, sieve it, dry it, and then wedge it just to make a cup. Now we can simply buy an amazing variety of premade and pretested clay bodies online. When glazing, we don’t have to find a feldspar deposit, mine it, test it, and process it. We don’t have to figure out all our own glaze recipes, we can simply search the Internet for recipes with detailed mixing and firing instructions or just purchase ready-made glazes in clean plastic bottles.

Developing your own glazes from natural materials has become much easier too, but you still get the satisfaction of making unique glazes that don’t come from neat plastic containers. Usually the hardest part is collecting and processing the natural materials. Thankfully this can now be done simply and easily.

I am sure you have noticed while driving on the roadways that roads have been cut through amazing rock formations many feet thick on almost any interstate. Smaller (and safer) secondary roads have them as well. These road cuts reveal the millions of years of deposition and metamorphosis in a single glance and can be accessed with little effort.

The only problem is identifying what rock you are collecting, especially if you don’t know much about geology. But this is easily overcome with an outstanding series of books called Roadside Geology. You can get these books for most states and they explain in detail the geology of the roadways in that state. For example you can get the Roadside Geology of Ohio or of Virginia, etc. It locates outcroppings and lists the type of rocks. Then all you have to do is stop and collect samples. These books are outstanding because they often also give you the history of the area. In the Roadside Geology of Ohio there is an excellent history of the pottery industry in eastern Ohio, how the rivers and natural deposits of clay and gas created this booming industry in the 1800s and an explanation of why it died.

I often collect natural glaze materials on various trips I take and then label and save the material until I have time to process it. The processing is actually pretty easy (depending on the material) if you are not in a hurry, and you have a bisque kiln and a ball mill. For example, clay or shale is pretty easy to process by slaking it down or ball milling, but processing feldspar can be a bit more challenging because the bonds in the rock are very strong. But even this is not that hard to do with a bisque kiln. For example: if I have a 5lb chunk of feldspar, normally what I would do it is to crush it with a sledge hammer until it was in small enough pieces and then use a hammer mill or a smaller hammer to grind it up. But an easier way is to just put it in a bisque bowl and run it through the bisque. Then, when it comes out, the feldspar will start to crumble. What is happening is that the bonds in the rock that have formed over thousands or millions of years are being broken with the heat and then not allowed to reform as the cooling is so quick, so the rock starts to fall apart. After the bisque firing, I usually see what I can knock off the big chunk and then if it is still hard underneath, I put it through the bisque again. Large chunks of rock possess a lot of mass so they don’t get completely heated in a quick bisque and you have to fire them again to heat the whole rock chunk. After I get it into smaller pieces, I put those in a ball mill for several hours. Then I screen the material from the ball mill and decide if I need to redo the larger chunks or simply throw them out.

There are other ways to find out where materials are other than the Roadside Geology book series. For example you may just come across an unknown outcropping at a local road cut. I often collect shale in Johnson City Tennessee on the east ramp onto State Route 381. It is where 381 and 36 (a.k.a. North Roan Street) intersect. I saw this outcropping which is just below a new General Shale Brick Headquarters. I did not know what type of shale this was so I searched on Google and found an abstract that showed a map of outcroppings of various types of shales in the area, many of which are on the roadways.

You can also search on the US Geologic Survey (USGS) or just go to your local office of the USGS and ask them for information on what is in your area. There are also offices of the US Soil Survey or Department of the Interior that may be able to help you. They will often have extensive maps of your area and will probably already know the composition of the clays and rocks in the area. If not, they can have them tested for you. Another good source for local rock and mineral deposits is a local university geology program. They may have academic papers on local rocks and minerals.

The outcropping of shale I collect can actually be looked up on Google Maps. You can actually see the black outcropping in a Google Earth image. You just look up “3103 North Roan Street, Johnson City, TN” on Google and then click “Maps.” (It is near Scully’s Bar and Grill and Ken’s Shopping Center.) Then take the little yellow person on the left zoom arrow and place him on the ramp to 381. That will make it turn into a photo and you can navigate up the ramp by clicking the white circle. When you get up a way and see the car, make the viewer turn to the right and you will see a black outcropping which is the Sevier Shale.

Other materials I collect include clays, limestone granite, gneiss etc. One really easy way that I find partially processed local materials is to head to the bottom of my driveway. I live in a rural area and have a gravel road and driveway. The “gravel” on the road is a ground granite that is mined and processed by Vulcan Materials, a stone supplier for road bond and rock. I contacted them and got a detailed analysis of their stone.

You can get this rock in varying sizes and mixtures. It is essentially ground up granite and if you have a driveway that is on a slope, the fine particles (silt) will wash down the driveway and deposit at the bottom just like it would in nature after thousands of years of erosion. It is actually a small alluvial deposit.

These found and processed materials can then be supplemented with store bought materials until you have the necessary ingredients to make a glaze. You can purchase whiting, dolomite, silica, or more unusual materials like rice hull ash, etc., whatever you need to give you a glass former, some fluxes, and some refractory materials.

If you still want unique materials to flux your glaze you can collect various types of wood ash. This is pretty easy if you have a wood stove. You can process specific types of wood simply by cleaning it out and then only burning one type of wood at a time. Plant and grass ash are a bit harder because you need to burn a lot and so that may be better done in a brick pit outside. (you want a brick pit because you don’t want other materials getting into the ash, like dirt from the ground or iron from a metal trash can, etc.)

Other ways of getting materials would be to use cullet or crushed glass bottles. Some people use pop or wine bottles. Brown bottles contain iron oxide while the blue ones contain small amounts of cobalt oxide. You can also find recyclers who process recycled florescent bulbs or medical glass. Another option would be to contact large suppliers for one pound samples of their materials, like pumice, which comes in various mesh sizes. For example, Hess Pumice ( sells Grade F, 325, ½ and FFFF. The Pacer Corporation, which mines Custer Feldspar, sells it in various grades (mesh sizes), e.g. 200 m, chips, etc.

Frank Hohenstein’s method is to use granite countertops cut-offs. He just gets the scrap pieces, crushes and sieves them into different mesh sizes, e.g. 100 m, 80 m, etc. Then he simply substitutes the crushed “granite” for the feldspar in the glaze recipe to give it a unique look. It turns out that “granite” countertops aren’t actually granite but rather various types of polished feldspars.

Once you have the materials, you can start to run basic melt tests, progression and line blends, or triaxial blends. These will give you a wonderful range of glazes from your natural materials with little effort.

Disclaimer: Prospecting for materials along the highway may be dangerous or not permitted. Attempt at your own risk and use caution. Find out the laws in your area before attempting any digging.

John Britt is the author of The Complete Guide to High-Fire Glazes. For more information about John and his work, see 

This article was excerpted from the April 2011 issue of Ceramics Monthly.